The invention disclosed herein is for obtaining uniform distribution of printer's ink on rollers that apply ink to photolithograph image plates such as are used in offset printing presses. A major problem in offset printing presses is to get uniform and consistent distribution of the ink film on the image plates. Non-uniformities result in ghosts appearing in the printed impression which are particularly noticeable in large areas that have dense coloration. In an attempt to minimize this problem, prior printing presses have arranged a large number of cylindrical rollers whose peripheries are in tangential contact with each other. The assembly of mutually contacting rollers is called an inker. At least one of the rollers is in tangential contact over its length with another roller that rotates in an ink bath and gets a heavy coating of ink on it. The next roller pressing against the roller in the ink bath, called the ductor roller, picks up a thinner coating of ink and transfers it to the series of rollers which, in turn, transfer and spread the ink from one to another until at the end of a series a thin and hopefully uniform coating is applied to the last stage of the rollers which are called the form rollers. The form rollers make tangential contact with the image plates on the plate cylinder and, thus, deposit an ink film of one color on the image plates. As the plate cylinder rotates, it transfers the impression to a blanket cylinder for further transfer of printing on the paper sheet or web. An impression cylinder presses the paper against the blanket cylinder.
A prior art practice for improving the uniformity of the ink coating on the form roller is to have at least one of the rollers in the series that contacts one or more of the other rollers oscillate in opposite axial directions which augments forming a uniform film of ink on the rollers. Until recently, oscillating ink rollers were driven axially by wobble plates and cams and swinging arms acting on the end of the shaft on which the rollers rotate. Because of the amount of space that the driving mechanism required, it was not always possible to locate the axially reciprocating rollers in the most advantageous position among the series of rollers.
The most advanced form of axially reciprocating ink roller is described in U.S. Pat. No. 4,509,426 which issued on Apr. 9, 1985 to the inventor in this application. The roller described in this patent rotates and slides axially in opposite directions on a stationary shaft. The mechanism for bringing about reversals in the axial direction of travel is entirely enclosed within the ink roller itself. In this patent, a cylindrical sleeve is fitted inside of the ink roller cylinder. About half the length of the interior of the sleeve contains a helical groove or thread which is like a left hand thread and the other half of the sleeve contains a right hand helix or thread. The ink roller is driven rotationally about its axis by a tangentially contacting adjacent roller which is power driven or receives its rotational force from other rollers in the inker series. Two prong elements are located within the internally threaded sleeve. One prong element is caused to engage the left hand helix so as to drive the roller in one axial direction to a predetermined limit. When this limit is reached, the first prong element is disengaged from the left hand helix or thread and the other prong element is engaged with the right hand helix or thread to cause the roller to reverse its axial direction abruptly until a repetition of the cycle takes place.
Rollers of the type described in the cited patent have, by way of industrial application, proved that they are capable of reducing ghosts and other artifacts by obtaining more uniform distribution of ink through the inker system down to the axially oscillating rollers. The patented axially oscillating roller is effective to smooth the ink on the image plates to eliminate the ink mottling which occurs with prior art rollers due to stickiness between the roller and plate. Use of the form rollers described in the patent has been limited, however, to relatively slow speed presses adapted to print on sheets of paper as opposed to a continuous web. Presses of this type pass sheets through at about 600 lineal feet per minute. Just by way of example, if the roller has a diameter of four inches, it will have a rotational speed of about 573 rpm. Assuming that the pitch of the right and left hand threads or helixes is 1/4", to obtain a shift of two inches to the left and back two inches to the right, sixteen revolutions of the roller will occur. And there will be about 72 direction changes per minute. A roller having an internally threaded sleeve can be designed with sufficiently low mass so that the almost instantaneous reversals do not set up noticeable shock or vibrations and do not put such strain on the parts that they break or wear out prematurely. However, it appears that the reversing roller described in the cited patent must be restricted to use in relatively slow presses in which the sheets to be printed move at not much more than 600 feet per minute.
Applying the patented oscillating roller design to inker systems used in printing presses which print on a continuous web instead of sheets would appear to be difficult if not impossible because the lineal speed of the web in the fastest presses at the present time can be around 2,000 feet per minute. Assuming the same design parameters as were assumed for the low speed press just discussed, the number of direction changes by the oscillating roller would be in the vicinity of 252 per minute. Even with the mass of the roller and the helical sleeve minimized, the inertia could be great enough to produce a noticeable shock and a concomitant tendency to damage the parts. Reversals are reduced by about 25% with the improved roller described herein.
The invention involves improvements in the reciprocating mechanism which make the autoreversing axially reciprocating ink roller usable in very high speed presses where many axial motion reversals occur in one second.